Field of the Invention
The invention relates to a thermal fuse for attachment to a circuit substrate, including a leaf spring formed from a narrow sheet metal strip having two ends with contact points at the ends, at least one of the contact points being a solder point for connection to corresponding circuit contacts of the substrate, and at least one spring arm being a link between the contact points. The invention also relates to a method for activation of such a thermal fuse.
Such fuses are used to protect circuit parts against excess thermal stress and can be used on any given circuits, e.g., on circuit boards. However, film and especially hybrid integrated circuits are the preferred area of application of the thermal fuse of the invention.
Thermal fuses of the kind mentioned above are commonly known, e.g., from German Published, Prosecuted Application DE-AS 1 15 624. In the fuse which is used therein for a resistor, the spring arm is pretensioned in its working position during installation of the fuse, i.e., the spring arm must be held in the pretensioned position by a special device during soldering and until the solder joint cools. That requires a complicated manipulation and costly equipment, especially in the case of film circuits.
In order to avoid soldering under tension, thermal fuses disclosed in German Patent DE 38 25 897 C2 and German Published, Non-Prosecuted Application DE 39 30 819 A1 propose holding spring arms of a fuse strap in their pretensioned position by means of a molded buttressing, so that no tightening or holding equipment is necessary during the soldering. However, the buttressing must be removed or bent free after the soldering, so that the tension of the spring arms can take effect. Furthermore, in most applications such very serviceable thermal fuses have a space requirement which is hardly available in many cases, due to their configuration with two spring arms being separated from each other and with a buttressing molded in between, so that they can only be used with difficulty, if at all, when space conditions are constricted.